Nuclear fuel assembly spacer and loop spring with enhanced flexibility

ABSTRACT

A spring-and-spacer assembly for maintaining fuel rods upright in a fuel assembly of a nuclear reactor core is provided. The spring is in the form of a continuous loop with first and second legs having mid-leg rod-contact regions. Bend regions are positioned on each side of each contact region extending toward the interior of the loop spring. In most positions, the spring is used to load two adjacent fuel rods. In configurations in which there is an unpaired fuel rod, a plate having a tab for contacting the spring is provided. The spring provides the desired force, such as about 2.5 pounds per fuel rod, in a small rod-to-rod spacing of less than about 0.14 inches.

FIELD OF THE INVENTION

The present invention relates to a spacer usable for positioning fuelrods and water rods in a nuclear fuel assembly and springs therefor,and, in particular, to springs that occupy reduced space in suchspacers.

BACKGROUND OF THE INVENTION

In a known type of nuclear power reactor, for example, a boiling-waterreactor, nuclear fuel is provided in elongated rods. The nuclear fuel istypically in the form of uranium oxide and/or plutonium oxide pelletsenclosed in zircaloy tubes. A number of such fuel rods are groupedtogether in an open-ended tubular flow channel. The flow channel withthe fuel rods positioned therein is referred to as a "fuel assembly" or"bundle." A plurality of fuel assemblies are removably positioned in thereactor core in a matrix. The reactor core formed in this manner iscapable of self-sustained fission reaction. The core is submerged in afluid, such as light water, which serves both as a coolant and as aneutron moderator.

The fuel rods in a fuel assembly are supported between upper and lowertie plates. The fuel rods are typically arrayed in parallel side by sidevertical upstanding relation. The fluid coolant flows past the fuel rodsin the intersticies between the vertical and parallel fuel rods. Toprovide proper coolant flow and preserve integrity of the fuel rods, itis important to maintain the rods in a preferred spaced relationship andto restrain them from bowing and vibrating during reactor operation.

To maintain spaced apart alignment of its fuel rods, a plurality of fuelrod spacers spaced along the length of the fuel assembly are providedfor this purpose. One type of spacer includes a plurality of generallycylindrical ferrule elements. An example of a spacer usable in a fuelassembly is that shown in U.S. Pat. No. 4,508,679, issued Apr. 2, 1989,to Matzner, et al.

As shown in Matzner, et al., one method of positioning a fuel rod withinthe ferrule elements of a spacer is to provide a spring member forbiasing the fuel rod against rigid stops in the ferrules. The springdepicted in U.S. Pat. No. 4,508,679 is in the form of a continuous loopof generally elliptical shape. The springs are positioned in the volumeof the spacer area between adjacent fuel rods.

Modern fuel bundle design typically includes fuel rods being arrayed ina square sectioned fuel bundle. The arrays originally where in a 7×7matrix. This left relatively large spaces between the fuel rods.Accordingly, the problem of placing springs between the rod to maintainthe rods in vertical upstanding relation presented a generally simplemechanical design problem.

Unfortunately, modern fuel bundle designs include much denser fuel rodarrays. Such arrays have gone from fuel rod matrices including 8×8 fuelrod arrays to 9×9 and 10×10 fuel rod arrays. This being the case, theinterstitial volume (or thickness) between the fuel rods has shrunk.Although the same spring action is required for the most part from thesprings acting with the spacers to maintain the fuel rods in verticalupstanding relation, the space in which such spring action can occur isvastly reduced. Previous fuel assemblies having 8×8 matrices fuel rodshad been constructed with rod-to-rod spacings (i.e.. distances betweenouter circumferences of adjacent rods) of about 0.160 inches (about 4mm). Modern fuel bundles are being designed with 9×9 matrices of fuelrods to have a reduced rod-to-rod spacing, such as about 0.12-0.14inches (about 3 to about 3.5 mm). This reduction in fuel rodinterstitial spacing has had severe constraints on the spring design.

It should be noted that the springs, although necessary for positioningthe rods, can have certain undesirable effects. These undesirableeffects include absorption of neutrons and interference with the coolantcirculation. Materials from which the springs are typically formedabsorb 20-100 times the number of neutrons absorbed by the spacermaterial. Accordingly, there is a high motive to maintain a minimum ofspring material within the fuel bundle.

SUMMARY OF THE INVENTION

A spring system is provided for bracing side by side fuel rods in dense(9×9) arrays. The spring system is incorporated to the several spacers,typically 6, 7 or 8 positioned at vertically distributed intervalsbetween the upper and lower tie plates in a square sectioned fuelbundle. The spacers and springs disclosed maintain a substantiallyuniform spacing between the vertically upstanding fuel rods and waterrods contained with the fuel bundle. A loop spring is provided as themain fuel rod biasing unit. Like the prior art, the loop springsurrounds portions of the spacer and is thus held in a generallyvertically disposed elongate loop by the spacer. Each loop spring actson typically a pair of fuel rods, one rod being on each side of thespring. Each fuel rod is confined within its own discrete ferrule at thespacer and is spring biased by the spring against paired protuberances.Each loop spring includes upper and lower C-shaped end portions forretaining the spring to the spacer. In between each upper and lowerC-shaped end portions there are formed two spring legs. The spring legsbegin at the ends of each "C" and flare with gradual reversing curvatureto substantially linear spring leg portions. Each spring leg includes inthe center of the leg a convex and outwardly disposed arched shaped rodcontacting portion for spring biased contact with the fuel rods oneither side of the spring. Unlike the prior art, two inwardly disposedU-shaped bends are incorporated in each spring leg immediately on eitherside of the outwardly disposed arched shaped rod contacting portion.These two U-shaped bends extend the effective length of the springmaterial over which the spring forces can act and effectively evenlydistribute the maximum bending forces on the spring material to thearched shaped rod contacting portion and the upper and lower C-shapedend portions. There results a spring which can provide a wide range ofdeflections while being confined to the narrow interstices betweenclosely spaced and dense fuel rod arrays such as the 9×9 arraydisclosed.

The disclosed 9×9 matrix fuel bundle has the seven middle latticepositions occupied by two relatively large and circularly sectionedwater rods. Seven central ferrules are removed to provide space for thewater rods. Two ferrules adjacent to the water rods are not paired withanother ferrule, so that the spring in each of these ferrules acts ononly one fuel rod. A special method for mounting these springs isrequired.

A water rod spacer plate is disclosed. This plate has two functions.First, it provides the required mounting for the disclosed loop springsin the locations where the spring bears on one rod only. Second, thewater rod spacer plate provides stops for the water rods.

The water rod spacer plate has a U-shaped central region for bearingagainst the two water rods at their adjoining exterior surfaces. Onelower portion of the "U" bears against one water rod; the other lowerportion of the "U" bears against the other water rod. These contactregions provide stops for the water rods. A spring mounted on a separateplate biases each water rod toward these stops.

The water rod spacer plate has a wing at each end of the U-shapedcentral region. One of these wings is shaped to provide mounting meansfor the springs which bear against only one fuel rod.

Because of the intervention of the water rods, at least one spring mustbe provided with a fuel rod on one side only. This spring isconveniently mounted on a disclosed interlock between a tab protrudingfrom the specially constructed fuel rod ferrule and a mating tab on the"U" sectioned water rod spacer member. Accordingly, required bias issupplied against all spaced apart vertical fuel rods and water rods ofthe dense 9×9 matrix fuel bundle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a broken-away partial elevational view, partly in section, ofa nuclear fuel bundle;

FIG. 2 is a plan view of a prior art previous fuel rod spacer;

FIG. 3 is an elevation view, partly in section, of the spacer of FIG. 2;

FIG. 4 is a perspective view of a loop spring provided in prior artspacers;

FIG. 5 is a top plan view of two adjacent ferrules and an interveningspring of prior art devices;

FIG. 6 is a front elevational view of the ferrules of FIG. 5, partly incross section;

FIG. 7 is an elevational view of a ferrule provided in prior artdevices;

FIG. 8 is a cross-sectional view of a ferrule taken along line 8--8 ofFIG. 7;

FIG. 9 is a top plan view of a spacer according to the presentinvention;

FIG. 10 is a top plan view of two adjacent ferrules and the associatedspring according to the present invention;

FIG. 11 is an elevational view, partly in cross section, of the ferrulesand spring of FIG. 10;

FIG. 12 is an elevational view of a ferrule according to the presentinvention;

FIG. 13 is a cross-sectional view taken along line 13--13 of FIG. 12;

FIG. 14 is a detailed view of a prior art spring mounted on a ferruletab;

FIG. 15 depicts the spring of the present invention mounted on theferrule ears;

FIG. 16 depicts the lower right quadrant of one spring according to thepresent invention, depicting certain forces thereon;

FIG. 17 is a partial top plan view of the spacer of the presentinvention;

FIG. 18 is a detailed top plan view of an unpaired ferrule of a spacerand the associated support plate;

FIG. 19 is a perspective view of a backup plate usable in connectionwith the present invention;

FIG. 20 is a partial exploded view of two adjacent ferrules and anassociated spring; and

FIG. 21 is a cross-sectional view taken along line 21--21 of FIG. 11.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The spring-and-spacer assembly of the present invention is provided foruse in connection with a fuel assembly, such as that depicted in FIG. 1.The fuel assembly 20 includes a plurality of fuel elements or rods 21,supported between an upper tie plate 22 and a lower tie plate 23. Thefuel rods 21 pass through a plurality of fuel rod spacers 24a, 24b,which provide intermediate support to retain the elongated rods 21 inspaced relation and to restrain them from lateral vibration. In oneembodiment, seven spacers are longitudinally evenly spaced along thefuel assembly.

Typically, the fuel bundle is in the order of eleven feet in length. Thematrix of fuel rods and water rods is in the order of 5 inches×5 inches.Typically, and in order to maintain the proper linearity between thefuel rods, water rods, and surrounding channel there are seven equallyspaced spacers between the lower tie plate at the bottom and the uppertie plate at the top.

Each of the fuel rods 21 is formed of an elongated tubular claddingmaterial containing fissile fuel and other materials, such as burnablenuclear poison, inert material, or the like. The fuel and othermaterials are sealed in the tube by upper and lower end plugs 26, 27.The lower end plugs 27 are registered and positioned in cavities 29formed in the lower tie plate 23. Similarly, the upper end plugs 26 fitinto cavities 31 in the upper tie plate 22.

Some of the fuel rods 21 may be provided with threaded lower and upperend plug extensions 27', 28' to receive retaining nuts 32. These fuelrods are known as "tie rods." In this manner, the upper and lower tieplates and fuel rods are formed into a unitary structure.

Typically, the fuel rod assembly includes a channel 33 of substantiallysquare cross section sized to form a sliding fit over the upper andlower tie plates 22 and 23 and the spacers 24, so that the channel 33may readily be remounted and removed. The channel 33 is fastened to apost 36 on the upper tie plate 22 by means of a bolt 37 passing througha tab 34. The lower tie plate 23 includes a nose piece 38 adapted tosupport the fuel assembly 20 in a socket in a core support plate (notshown) in the reactor pressure vessel. The end of the nose piece isformed with openings 39 to receive the pressurized coolant so that itcan flow upwardly among the fuel rods.

One or more of the fuel rods 21 may be replaced by a moderator tube orwater rod 41 which contains water, a neutron moderator. The moderatortube 41 may be apertured, as shown at 42 and 43, and the upper and lowerend plugs may be formed with passages 44, 46 to permit flow of watermoderator therethrough.

An understanding of the present invention is promoted by a briefdescription of the spacer and spring apparatus of previous devices. Aprior art spacer 24, as depicted in FIGS. 2 and 3, is made up of aplurality of substantially cylindrical ferrules 51 joined to oneanother, for example by welding, at the upper and lower edges ofabutting portions of adjacent ferrules. Each of the ferrules 51 providesa space for a fuel rod 21 or moderator tube 41.

Preferably, each of the ferrules 51 is formed of a short section oftubing having circular cross section. A peripheral band 56 surrounds andsupports the plurality of ferrules 51. Two stops 62a, 62b are providedin each ferrule. Preferably, the stops 62a, 62b are formed integrallywith the ferrule 51 as laterally spaced pairs of arched portions of theferrule walls near the upper and lower edges of the ferrule.

Preferably, the stops 62a, 62b are laterally oriented to minimize spacerprojected area and coolant-flow resistance. As best seen in FIG. 3, eachpair of ferrules 51 shares a continuous loop spring 70. As depicted inFIG. 4, the continuous loop spring 70, used in previous devices,includes first, second, third, and fourth legs 72a, 72b, 72c, 72d joinedby mid-positioned ridges or convex arcuate spring contacting portions74a, 74b and end-positioned arches or C-shaped portions 76a, 76b. Thesprings 70, depicted in FIG. 3, are in their substantially unstressed orunflexed condition. The springs in the flexed or stressed condition aredepicted in FIGS. 5 and 6.

In the previous devices, depicted in FIGS. 5 and 6, the springs 70occupy a region between two adjacent fuel rods. In previous devices, therod-to-rod spacing 78 was approximately 0.16 inches (about 4 mm). Asseen in FIG. 6, the previous spring 70 contacted the fuel rods 82a, 82bat the ridge portion 74a, 74b, and provided a force tending to positionthe fuel rods 82a, 82b against the stops 62a, 62b of each ferrule.

Each ferrule 51 was provided with a C-shaped slot 84, defining a tab 86.In assembly, the spring 70 was inserted into the slot 84 and positionedover the tab 86. An adjacent ferrule was fitted to the initial ferrule,with the C-shaped slot of the second ferrule oriented with the tab 86pointing in the opposite direction from the tab of the first ferrule.The tabs 86 of the first and second ferrules overlapped each other. Thespring was then captured between the two ferrules, and formed a looparound the overlapped tabs 86.

A number of difficulties have been noted in connection with the previousspring-and-spacer assembly, particularly when such assembly is intendedfor use in connection with a fuel assembly having a reduced rod-to-rodspacing. The spring which is used in a spring-and-spacer assembly mustprovide the required amount of force, preferably about 2.5 pounds (about1 kg), but must also have sufficient flexibility to tolerate deflectionbeyond that normally needed for positioning the fuel rod withoutsubstantial permanent deformation. Deflection of the spring beyond thatnormally needed for positioning the fuel rod can occur, for example,during shipping or assembly, particularly if the fuel rod 82 is encased,during assembly, in a protective plastic sheath (not shown). As shown inFIG. 6, the previous loop spring 70 could deflect only a limiteddistance before the interior surface of the spring 70 would strike thetab 86 of the ferrule. This limitation places close tolerances on theconfiguration of the spring 70, particularly when a smaller rod-to-rodspacing 78, such as about 0.12 to 0.14 inches (about 3 to 3.5 mm) isdesired.

FIG. 9 depicts a spacer and the associated springs according to thepresent invention. The springs 92a and 92b are arranged such that asingle spring 92a loads the fuel rods positioned in two adjacentferrules 94a, 94b. The overall length 95 (FIG. 20) of the spring is lessthan the length 97 of the ferrules 94a, 94b.

FIGS. 10 and 11 depict a portion of a spacer and the associated springsaccording to the present invention. A spring 92a is formed of a metallicribbon having a width 96 (FIG. 20) and a thickness 98 (FIG. 16). Thespring can be formed of a number of materials having suitable strength,corrosion resistance, and resilience characteristics. In one preferredembodiment, the spring is formed of a nickel alloy, such as Inconel,available from Huntington Alloy Products Division, International NickelCo., Inc., Huntington, W. Va. In the preferred embodiment, the width 96is about 0.1 to about 0.15 inches (about 2.5 to 3.8 mm), and thethickness 98 is about 0.01 to 0.015 inches (about 0.25 to 0.38 mm). Theribbon is formed into a continuous loop, i.e., a shape with a crosssection topologically equivalent to an annulus. Preferably, the springhas at least two planes of symmetry, a longitudinal mid-plane 102 and alateral mid-plane 104 (FIG. 15). Thus, the spring has four congruentsectors, the lower-right sector being depicted in FIG. 16.

The longitudinal extent of the spring can be considered as having sevensections, as shown in FIG. 15. A ridge or arch-shaped rod-contactingportion 106 is formed in each leg, centered about the lateral symmetryplane 104 at the mid-span of each leg. Disposed on either side of thearch section 106 are U-shaped bend portions 108, 112 extending in adirection generally toward the longitudinal mid-plane 102. Upper andlower leg portions 114, 116 are positioned adjacent the bend portions108, 112, respectively. C-shaped end portions 118, 122 are adjacent theleg portions 114, 116.

The U-shaped bend portions 108, 112 constitute the departure from theprior art which makes the disclosed spring design possible.Specifically, U-shaped bend portions 108, 112 are formed adjacent toeach of the two arch-shaped rod-contacting regions 106. Viewed from theexterior of the spring, the bend portions 108, 112 are concave, i.e.,they extend toward the interior of the loop spring. The bend portions108, 112 are on the loop interior side of the planes defined by theadjacent, substantially planar upper and lower leg portions 114, 116.

These two U-shaped bend portions 108, 112 in each of the spring legseach provide two functions for the improved spring design. First, theyprovide additional spring length, which increases the flexibility of thespring.

Secondly, these same two U-shaped bend portions 108, 112 cause themaximum bending stress to be evenly distributed between the high stressregions of the spring.

A first region where the bending stress is high is the convex andoutwardly disposed arch shaped rod contacting portion of the spring 106.This member has effective maximum compression forces acting on theoutside of the spring member at this juncture and maximum tension forcesacting on the inside of the spring member. A second region where thebending stress is high (and oppositely disposed) is in the upper andlower C-shaped members 118, 122. In these members the bending stress isthe opposite with maximum tensile forces on the outside and maximumtension forces on the inside.

It will be understood that the disclosed design has the benefit ofequalizing the bending stress in the respective two regions. Thisequalization of bending stress provides the maximum spring deflection.

The shape of the spring of the present invention 15 can be contrastedwith that of the previous devices depicted in FIG. 14. The width 124' ofthe previous design was greater than the end portion width 124 of thepresent design. The previous design, shown in FIG. 14, did not includeU-shaped portions 108, 112 adjacent to the rod-contacting region, sothat the previous design spring had a rod-contacting portion 106' whichwas immediately adjacent to the substantially planar upper and lower legportions 114', 116'. In the previous design, depicted in FIG. 14, thespring 92' encircled and accommodated a double thickness of the tabs126, 128 formed in adjacent ferrules, since these were positioned in anoverlapping configuration.

In contrast, in the present invention, only a single tab thickness 132must be accommodated by a spring, since the tabs meet in a butt-jointfashion, as described more fully below.

Furthermore, the tabs 126, 128 of the previous design, depicted in FIG.14, were longitudinally continuous. In the present design, depicted inFIG. 15, there is a space between the upper ear 132 and lower ear 134formed, as described more fully below, by the middle leg 178 (FIG. 12)of an E-shaped cutout 158, which assists in accommodating the U-shapedbend portions 108, 112, particularly in the flexed position, as depictedin FIG. 11.

Both the previous spring design, depicted in FIG. 14, and the presentspring design, depicted in FIG. 15, are loaded by forces 136', 136,respectively, where the springs 92', 92 contact the fuel rods. In theprevious design, depicted in FIG. 14, some flexibility is provided bythe arch-shaped projections in the rod-contacting region 106' and by theprojections 138a, 138b, 138c, 138d, and horizontal portions 142a, 142bof the end regions 118', 122'. In the spring of the present invention,as depicted in FIG. 15, the bend portions 108, 112 provide increasedflexibility, as compared to the prior art spring.

In addition to increasing the spring flexibility, the bend regions 108,112 provide a more desirable stress distribution. In the spring ofprevious designs, depicted in FIG. 14, the largest stresses on thespring occur near the spring contact portion 106' and in the center ofthe end portions 142a, 142b. Finite element computer analysis of theprevious spring designs of FIG. 14 shows that the stress at mid-span106' is greater than the stress at the ends 142a, 142b. The ends 142a,142b have greater local flexibility than the mid-span region 106'. Thisgreater flexibility at the ends reduces the stress near the ends 142a,142b, and increases the relative stress at mid-span 106'. Adding bends108, 112 near the mid-span 106 provides greater local flexibility nearthe mid-span 106, which would be more nearly equal to the flexibility ofthe end regions 118, 122. This is supported by the results of a computermodel analysis of the spring 92.

Because of the symmetry of the spring, noted above, it can be analyzedby considering one quarter of the spring, as depicted in FIG. 16.Stresses at the mid-span 106 and the end point 144a are proportional tobending moments. Equilibrium of bending moments can be expressed as

    M.sub.144a +M.sub.106 =(F/2)×(L/2)                   (1)

where M_(144a) is the bending moment at the lower end mid-point 144a,M₁₀₆ is the bending moment at mid-span 106, F is the total load on thespring, and L is the length of the spring. The relationship expressed inequation (1) is independent of the shape of the spring between themid-span point 106 and end point 144a. It is desirable to provide aspring which has equal bending moments and equal stresses at these twopoints, 144a, 106. Finite element analysis confirms that stresses atmid-span 106 and the end point 144a are nearly equal for the new springdepicted in FIGS. 15 and 16. Accordingly, the stress distribution of thenew configuration is more desirable than the stress distribution of theold configuration, depicted in FIG. 14.

Equal stress gives a more efficient structure. It is desired to getmaximum deflection at a prescribed load; local yield of material limitsthe load. Two stresses equal to one another provide maximum deflectionat the yield stress of the material.

It will be realized that making the spring longer cannot practically beconsidered because of the constraints of the spacer height. The designhere disclosed gives an effectively longer overall length within thelimited spacer height.

The spring of the present invention is used in connection with a spacer(FIG. 9), comprising a number of ferrules 94 (FIGS. 10-13). The spacercan be formed of a number of materials having a suitably low neutronabsorption cross section, preferably a zirconium alloy, such asZircaloy-4. The spacer in one embodiment is square-shaped with a sidelength of about 5.25 inches (about 13.3 cm), and the ferrules are about0.57 inches (about 16.2 mm) in outside diameter, with a wall thicknessof about 0.02 inches (about 0.5 mm).

The ferrule of the preferred embodiment includes two upper stops 146,148, and two lower stops 152, 154 extending inwardly into the ferrule94. Preferably, the stops 146, 148, 152, 154 are formed by indentingportions of the ferrule wall to produce inwardly-arching structures.

As best seen in FIGS. 10-13, the fuel rods 155a, 155b are abuttedagainst the stops 146, 148, 152, 154 to place the fuel rods 155a, 155bin a preferred position within the ferrule 94, such as a positioncoaxial with the ferrule. The force to maintain the fuel rods 155a,155b, in contact with the stops 146, 148 152, 154 is provided by thespring 92a.

In order to provide such force, the spring 92 is mounted on ears 132,134 of the ferrule 94. (See FIG. 12). The ears 132, 134 are defined byan E-shaped slot 158, formed in the wall of the ferrule 94. The upperand lower legs 162, 164 of the E-shaped slot 158 are shown in ferrule94.

The middle leg 178 of the E-shaped slot 158 serves to define the ears132, 134. Each of ears 132, 134 has an edge surface 135, 135', an innersurface 137, 137', and an outer surface 139, 139' (FIG. 20).

As seen in FIGS. 10 and 13, the tab 176 is curved outwardly from thecircumference of the ferrule 136. As seen in FIG. 20, the spring 92 isattached to the ferrule 136 by slipping the spring over the upper andlower ears 132a, 134a of a first of the ferrules 94a.

The spring is retained in its position on the ears 132a, 134a bypositioning the corresponding ears 132b, 134b (FIG. 20) of the nextadjacent ferrule 94b in a butt-joint relationship with the tab and ears176a, 132a, 134a of the first ferrule 94a, facing in the directionopposite that of the tab and ears 176b, 132b, 134b of the second ferrule94b.

The corresponding ears 132a, 132b and 134a, 134b on the two ferrules94a, 94b are thus configured with their edges 135, 135', adjacent, butwithout any overlapping, i.e., without substantial contact of, the innerand/or outer surfaces 137, 137', 139, 139'. To provide for thisbuttjoint contact, the width 179 of the tab 176 is less than the width179' of the corresponding tab 86 (FIG. 7) of prior art ferrules.

As seen in FIG. 10, the tabs 176a, 176b are curved in oppositedirections (e.g. concave and convex, respectively, when viewed from theinterior of the first ferrule 94a), which results in a tendency tocenter the spring 92 on the tabs 176a, 176b. The ferrules are held inthe position depicted, by welding, preferably at the top and bottomareas of the ferrules which are in contact. A peripheral band 180surrounds and supports the plurality of ferrules (FIG. 9).

The spacer shown in FIG. 9 has a central region 181 where the ferrulesare omitted. This space is used for moderator tubes. FIG. 17 shows thecentral region in more detail. Water rods W1 and W2 occupy the centralregion. With the spacer configuration of FIG. 9 there is an even numberof ferrules; 72 ferrules. However, it is not possible to form pairs withall of these ferrules. FIG. 17 shows two ferrules, 94c and 94d, whichare not paired with other ferrules.

As shown in FIGS. 10 and 11, the spring loads two adjacent fuel rods. Ifone rod is absent, the spring will no longer provide the required loadon the remaining fuel rod.

In order to provide proper spring force for the unpaired ferrules 94c,94d, restraints for the corresponding springs 92c, 92d are provided. Asshown in FIG. 17, spacer support plates 184a, 184b are attached to thespacer in the region of each unpaired ferrule 94c, 94d. As best seen inFIG. 19, the plate 184 is provided with a central U-shaped member 190.This member defines two shoulder 191, 192. As seen in FIG. 17, members191 and 192 bear against the respective water rods W1 and W2 in thecentral portion of the fuel bundle. Since one of these U-shaped membersbears on the water rod pair equally and from opposite sides, the twowater rods W1 and W2 are effectively forced apart.

This forcing of the water rods away from one another is opposed.Specifically it is opposed by springs 195 at water rod W1 and spring 196at water rod W2. A positioning of the water rod by the spacer which isprecisely analogous to that of the fuel rods occurs.

It can be seen that the U-shaped member 184 is provided with wingmembers 197, 198. These respective wing members form the points ofattachment to ferrules adjacent the respective water rods.

It will be understood that with the placement of the water rods W1 andW2, some of the respective fuel rods will be lacking the normal offsetting fuel rod on the opposite side of the disclosed spring. Thisbeing the case, plate 184 is provided with ears 134', 136' forsupporting the springs 194 and 196 in a manner similar to the ears 132,134 in a normal ferrule. The region between the ears 132', 134' isoccupied by a backup tab 186. The tab 186 is bent outwardly to contactthe exterior of a rod-contacting portion 106 of the spring 92c (FIG.18). The backup tab 186 thus substitutes for the missing adjacent fuelrod, and provides a restraint on the spring 92c.

In light of the above description, a number of advantages of the presentinvention are apparent. The spring can be provided in a smaller space,such as that available with a rod-to-rod spacing 78' (FIG. 10) ofbetween about 0.12 and 0.14 inches (about 3 to about 3.5 mm), and yetcan produce the required force for fuel-rod loading, preferably about2.5 pounds (about 1 kg). The present spring is more flexible thanprevious springs, and has a more advantageous distribution of stresses,with the mid-span stress being approximately equal to the end stress.The spring-and-spacer design provides for desirable coolant flow nearthe rod. The spring-and-spacer assembly provides for ease ofconstruction. The spring is adaptable for use with unpaired ferrules byproviding a plate with a backup tab.

A number of variations and modifications of the present invention willbe apparent to those skilled in the art. The spring and/or spacer can bemade of materials other than those discussed herein. The general springand spring-and spacer assembly configuration can be used in connectionwith spacers having more or fewer fuel rod positions than those depictedherein. Various aspects of the disclosed design can be usedindependently of other aspects, for example, a spring can be providedwith bend regions, but without the butt-joint configuration of theferrule tabs.

Although the description of the present invention has included adescription of a preferred embodiment and various modifications thereof,other modifications and variations will be apparent to those skilled inthe art, the present invention being described in the following claims.

What is claimed is:
 1. A loop spring for maintaining fuel rods in spacedapart relation in a fuel bundle spacer, said spacer including; first andsecond overlying spring supporting elements for supporting a spring,said spring supporting elements disposing said spring on opposite sidesto and towards a ferrule for containing fuel rods, said spring elementsof the type formed from a continuous elongated loop of spring material,said spring element in the unstressed state having said loop defining anupper C-shaped end in said loop for surrounding and being supported fromsaid first spring support member from said spacer; a lower C-shaped endin said loop for surrounding and being supported from a second andunderlying spacer member; paired spring leg members on either side ofsaid loop for closing said loop between said C-shaped ends; said pairedspring leg member further defining medially thereof a convex andoutwardly disposed arch shaped rod contacting portion, said rodcontacting portions on each of said paired spring leg members beingmedially located in said spring leg members between said upper and lowerC-shaped sections; the improvement in said loop spring membercomprising:two inwardly disposed U-shaped bends formed in either springleg, said U-shaped bends being joined to and on either side of saidconvex and outwardly disposed arch shaped rod contacting portion, eachsaid U-shaped bend being immediately adjacent said convex and outwardlydisposed arch shaped rod contacting portion at one end and attached tosaid spring leg at the other end, each rod containing portion and saidU-shaped bend on each side of said rod contacting portion definingoffsets in the unstressed state of said spring from paired planes oneach side of said loop spring, each said paired planes generally definedby and including the substantially linear portion of said paired springleg members beginning at said C-shaped ends and including said pairedspring leg members and ending at the beginning of said U-shaped members,said plane being defined across said rod contacting portion and saidU-shaped bends on either side of said rod contacting portion so thatsaid rod contacting portion is offset on one side of said plane awayfrom the opposite side of said spring and said U-shaped bends on eitherside of said rod contacting portion are offset on the other side of saidplane to and toward the center of said loop spring whereby said U-shapedbends in the stressed state of said spring expand the effective lengthof said spring and equalize bending stresses between said two convex andoutwardly disposed rod contacting portions and said C-shaped ends. 2.The invention of claim 1 and wherein said rods separated by said spacerare confined in a 9×9 matrix.
 3. The invention of claim 1 and includinga first fuel rod ferrule for containing a fuel rod on one side of saidspring and a second fuel rod ferrule for containing a second fuel rod onthe opposite side of said spring.
 4. A spacer assembly for maintaining amatrix of fuel rods in spaced apart relation between upper and lower tieplates said spacer comprising:a matrix of side by side ferrules eachsaid ferrule corresponding to a corresponding matrix position of saidfuel rods, said ferrules configured for encircling said fuel rods atsaid spacer; each said ferrule defining four inwardly disposed fuel rodcontacting points said fuel rod contacting points for abutting fuel rodsurged against said points; paired said ferrules defining upper and lowerspring support points for supporting said spring between said ferrules;at least one loop spring for maintaining fuel rods disposed in adjacentferrules in spaced apart relation; said loop spring formed from acontinuous elongated loop of spring material; said spring material inthe unstressed state being of the type having an upper C-shaped end insaid loop for surrounding and being supported from a first spacersupport member; a lower C-shaped end in said loop for surrounding andbeing supported from a lower end and second spacer supporting member;paired spring leg members connecting said top and bottom C-shaped endsto form said elongated loop shaped spring member into a continuous loopsuspended from said spring support members; said paired spring legmembers each further defining medially thereof a convex and outwardlydisposed arch shaped rod contacting portion, said rod contacting portionbeing medially located in said spring members between the upper andlower C-shaped members; said spring leg member further defining twoinwardly disposed concave U-shaped bends formed in each spring leg, saidconcave U-shaped bends being immediately joined to and on either side ofsaid convex and outwardly disposed arch shaped rod contacting portion atone end and attached to said spring leg portion at the other end, eachrod contacting portion and said U-shaped bend on each side of said rodcontacting portion defining offsets in the unstressed state of saidspring from paired planes on each side of said loop spring, each saidpaired planes generally defined by and including the substantiallylinear portion of said paired spring leg members beginning at saidC-shaped ends and including said paired spring leg members and ending atthe beginning of said U-shaped members, said plane being defined acrosssaid rod contacting portion and said U-shaped bends on either side ofsaid rod contacting portion so that said rod contacting portion isoffset on one side of said plane away from the opposite side of saidspring and said U-shaped bends on either side of said rod contactingportion are offset on the other side of said plane to and toward thecenter of said loop spring whereby said U-shaped bends in the stressedstate of said spring expand the effective length of said spring andequalize the bending stresses between said two convex and outwardlydisposed rod contacting portions and said C-shaped ends.
 5. The spacerassembly of claim 4 and wherein said spring extends at one spring legmember into a first ferrule and at a second spring leg member into asecond ferrule.